Recently there are increased demands for more uniform electroplating, above all in the electronics industry. For instance, the printed-circuit board has a width of the conductor and a gap between the neighbouring conductors far less than 100 .mu.m in order to attain higher integration density.
In order to comply with such demands, it is necessary to lower a metal ion concentration in the electroplating solution and consequently a cathode current density so as to form smaller matallic crystals.
As well known by those skilled in the art, when the concentration of copper sulfate is in the range of 60-120 g/1, the cathode current density ranges from about 1A/dm.sup.2 to about 3A/dm.sup.2.
If the concentration is lowered down to the lower limit, 60g/1 for the purpose referred to above, the current density necessary for preferable electroplating is to be in the range of 1-1.5A/dm.sup.2 according to experiences by the inventors, which means that it takes so much time for electroplating in a desired thickness e.g. 25-30 .mu.m.
It has been well known that when the electroplating bath is relevantly stirred it is possible to increase the cathode current density by two or three times, and consequently it has been proposed and actually used to utilize air bubbles for causing rising currents and stirring of the solution, which will be explained in reference to FIGS. 5-8 so that the problems of the technical standard of this technical field may be readily understood.
In an electroplating bath 11 substantially of rectangular parallelepiped, which is fully filled with an aqueous copper sulfate solution 12, there are arranged a substrate 21 to be vertically held as a chatode plate and a pair of anode plates 22, 22' also held vertically so as to put the substrate 21 therebetween with respectively leaving a substantially same distance. There is mounted a bubble box 13 at the bottom of the bath.
The bubble box 13 extending along the bath side wall and shown in FIG. 5 is formed with two rows of nozzles 13a, 13a' (see also FIG. 8) in the upper wall thereof so as to release bubbles which may rise up in the solution along the both sides of the substrate 21, whereby a pair of convection currents shown by arrows are formed. Owing to rising currents, the level of the solution is swelled up above the substrate as more or less exaggeratedly shown by phantom lines.
In order to realize homogeneous and symmetrical electroplating of copper on the both sides of the substrate, it is necessary to keep the smooth and stable convection currents of the bath solution, which are obviously formed on a balance of various factors.
When the balance is once lost even a little, for instance the convection current around the right anode plate 22' (FIG. 6) is made stronger, the current descending along the outer side of the anode plate 22' down to the lower edge thereof and just rising up along the inner side thereof is apt to push bubbles just released from the right nozzles 13a towards the left so that stirring of the solution at the right side of the substrate 21 as the cathode plate is weakend whereby the so called burnt deposit is caused on the corresponding surface thereof.
When such state is kept for a while the liquid pressure is increased relative to that in the right side, which makes the left convection current to be stronger relative to the right one so as to cause burnt deposit on the left side surface of the substrate. The repetition of the above can not ensure relevant electroplating.
Now in reference to FIG. 8 which is a section of the bath 11 taken along a line VIII--VIII in FIG. 7 so as to show only one side, i.e. the left side of the substrate 21 in FIG. 7 as in a side elevation, bubbles released through the nozzle or nozzles 13a formed in the middle portion along the box rise up in the solution without being influenced by the inner surfaces of the side walls of the bath 11 and with pulling bubbles released through the outer holes 13a so as to form a more strongly rising current which may make the solution level to be swelled up at the center as shown by phantom lines.
The rising current is then divided to flow leftwards and rightwards and further to form two descending currents as shown by arrows. Since there are little or no bubbles at the left and right portions in the solution as shown in FIG. 8, burnt deposits may be formed on the substrate near the left and right edges thereof.